PARMOD.TM. technology involves the use of novel PARMOD.TM. compositions which can be easily printed or deposited on electronic components and cured at low temperatures to form a highly conductive, well bonded, well consolidated pure metal component. Using PARMOD.TM. technology, the hazardous waste production characteristic of conventional photolithography, plating and etching processes is completely eliminated. Basic PARMOD.TM. technology has been disclosed in U.S. Pat. No. 5,882,722 issued on Mar. 16, 1999 to Paul Kydd et al., and PCT Application WO 98/37133, "Low Temperature Method And Compositions For Producing Electrical Conductors" of Paul Kydd, et al., published on Aug. 27, 1998. The essential constituents of PARMOD.TM. compositions are 1) a metal powder or metal powder mixture of specified characteristics and 2) a Reactive Organic Medium (ROM) in which the consolidation of the metal powder mixture to a solid conductor takes place.
The metal powder mixture may comprise a mixture of at least two types of metal powders including: metal flakes; metal powders; and colloidal or semi-colloidal metal powders. The ROM can consist of any metallo-organic compound which is readily decomposable to the corresponding metal at a temperature below 400.degree. C., or an organic compound which can react with the metal to produce such a compound. Examples include metal soaps and the corresponding fatty acids, metal amines and metal mercapto compounds and their corresponding amino and sulfide precursors.
The constituents of these compositions are weighed out in appropriate proportions, mixed with additional surfactants or viscosity modifiers, if needed to provide the proper consistency, and milled together, for example, on a three roll mill, to provide a homogeneous, printable composition.
The PARMOD.TM. composition is applied to the electronic component using any convenient technology. Screen printing and stenciling are suitable for rigid substrates in relatively small numbers with high resolution. Gravure printing, impression printing and offset printing are suitable for high production rates on flexible substrates. Ink jet printing and electrostatic printing offer the additional advantage of direct computer control of the printed image. This permits circuits to be printed directly from Computer Aided Design (CAD) files and eliminates the need for special tooling; thus, each circuit can be different, if desired, for coding or prototyping. The same end can be achieved at lower production rates with computer-controlled dispensing equipment. This equipment produces dots or lines by moving a needle over the surface and dispensing printing composition supplied by a pump or pressurized syringe.
The PARMOD.TM. compositions are then cured by exposure to heat for a short period of time. This time varies with the temperature to which the substrate can safely be exposed, but is less than a minute to achieve most of the electrical conductivity of which the composition is capable; in some cases the PARMOD.TM. composition is cured in less than 10 seconds at temperature.
Silver and gold may be cured in air. Copper and other non-noble metals require a protective atmosphere. Nitrogen with less than about 3 parts per million of oxygen has been found suitable for processing copper compositions. Addition of water vapor during the curing process, but not before or after, has been found to be beneficial in curing copper compositions.
A typical class of electronic components for which the PARMOD.TM. process is particularly useful are substrates for electronic circuitry. Substrates to which these compositions can be applied include rigid, glass-reinforced epoxy laminates, polyimide films for flexible circuits, other polymer-based electronic components, metal pads and semiconductor components.
Adhesion of PARMOD.TM. compositions to metals generally requires a clean metal surface, similar to the requirements for soldering. Acid constituents in the ROM act as fluxes to promote adhesion. Plating or tinning the metal pads is also effective. The use of organic solder protectants on copper pads is effective. Adhesion to semiconductors requires metallization with which the compositions are compatible.
As the variety PARMOD.TM. compositions has expanded to encompass different metals, different ROMs, and different substrates it has become evident that certain combinations of PARMOD.TM. and substrate result in decreased adhesion of the PARMOD.TM. to the substrate and lower conductivity of the cured PARMOD.TM. conductor. For example, copper PARMOD.TM. does not cure properly on epoxy surfaces even though the minimum cure temperature of 260.degree. C. is within the capability of FR-4. The epoxy appears to exert some deleterious effect on the curing PARMOD.TM. mixture.
It is an object of this invention to provide an adhesive barrier coating on commonly used printed wiring board substrates which will allow PARMOD.TM. images to cure properly and bond securely to the substrate.